When shady characters being ques tioned by federal agents make seri ous charges against otherwise respectable citizens, the least authorities can do is keep the allegations confidential — or back them up with facts.

Because if those charges prove false, then publicizing them amounts to nothing less than a vicious smear, leaving the accused fighting to save his reputation.

Witness what happened to storied ex-cop and private detective Bo Dietl — who last week found himself accused, in a leaked FBI report, of having sold secrets to the Gambino crime family.

So, at least, claimed mobster-turned-informant John Alite (hardly an impeccable source), who named 10 other cops, too.

Dietl fervently denied it. And he’s backed up by former top mob prosecutor Gregory O’Connell, who’s put numerous area wiseguys behind bars and told The Post that no one ever suggested Dietl was on the take.